A kinase which phosphorylates the 20,000 dalton light chain of platelet myosin has been isolated from pre-fusion rat myoblasts, human rhabdomyosarcoma cells and SV-40 transformed astrocytic neuroglial cells. The native kinase of each of these cells phosphorylates the myosin of its own cell also. These kinases differ from the myosin light chain kinase found in skeletal muscle in not requiring Ca 2 ions for activity and in having different substrate requirements.